Interest in converting inanimate surfaces such as walls, floors, windows, and the like into sensate surfaces (e.g., surfaces that can locate and characterize object contact, impacts or proximity) has recently grown. The heightened interest is, in part, a result of increased attention to large-scale interfaces for human-computer interaction in smart homes, public spaces, workplaces, etc. Generally, a sensate surface is created with the addition of transducers on, adjacent to, or integral with the surface. The transducers track the positions of objects on or adjacent the surface.
Much development has gone into the production of “touch screens,” a common example of an interactive surface that tracks the position of fingers against a video monitor. The technologies in touch screens (resistive sandwiches, active acoustic absorption tracking, capacitive sensing, infrared LED-receiver grids, for example) generally do not scale gracefully to large interactive surfaces (e.g., of over a meter in length), because of complexity and expense. Large video displays (e.g., from video walls and projections) are already becoming common in public spaces, but are not yet interactive, partly because of the difficulties inherent in reliably and economically scaling the appropriate technologies for tracking hands across the active surface.
Impact-locating systems are one current area of sensate-surface development. Some impact-locating systems identify the location of a surface impact by analyzing the resulting acoustic signal. This approach is attractive because of its simplicity, economy, and minimal interference with the surface (a set of contact transducers may be glued onto the surface to receive the acoustic wavefront from the impacts); people can simply knock or tap on the surface to interact. Glass is a ubiquitous material used for large surfaces in present-day construction. Because glass is transparent, it can be used to dynamically display information behind the surface, and is therefore an attractive material for use as a sensate surface. As all sensors can be mounted on the inside surface of a single-paned sheet of glass, it is well-suited for instrumenting outside-facing windows; e.g., it forms a straightforward, secure, and practical means for making shop windows interactive for passers-by outside. However, acoustic signal propagation in glass is characterized by dispersion, i.e., different frequencies propagating at different velocities, creating the tendency of the signal to spread out as it travels along the glass surface.
The dispersive nature of acoustic signals in glass is problematic for impact-locating systems because the time of arrival of the acoustic signal at each transducer location is difficult to determine. For signals originating from an impact on glass, dispersion is generally seen as a variable amount of low-amplitude higher-frequency acoustic signals that often travel ahead of the main wavefront. The dispersion results in an acoustic signal whose characteristics vary as the distance from the impact increases. Thus, the acoustic signal characteristics received at each transducer will vary from one another unless the transducers are precisely equidistant from the point of impact. Additionally, the frequency components created by an impact can vary, depending upon the object that strikes the surface, causing a corresponding variation in the primary acoustic propagation velocity through dispersion. Thus, a specific propagation velocity cannot be assumed when determining the impact coordinates unless the impact source is known or characterized.
In theory, the differential time of arrival of the acoustic signal between each of at least three transducers allows an impact location to be determined. One such approach is described by Paradiso et al. in “Sensor Systems for Interactive Surfaces,” IBM Systems Journal, Vol. 39, Nos. 3 & 4 (2000). In accordance with the approach described therein, impact-location coordinates are mapped based upon the differential timings of the acoustic signal received at each of four transducers. However, because the differential time of arrival must be determined, the leading edge of the acoustic signal must be accurately identified so that the signal arrival time at each transducer is detected consistently. In practice, the approach described in the above-mentioned article can fail to accurately locate the impact coordinates when signal dispersion prevents accurate identification at each transducer of both the acoustic signal wavefront and the corresponding acoustic signal time of arrival. Additionally, the preceding approach fails to detect low-frequency acoustic signals created by certain impact sources, e.g., a fist, and can be vulnerable to interference from ambient noises.